Bipolar junction transistors (BJTs) are a type of transistor used in electronics and signal processing circuits. They are composed of two pn-junctions in a three-terminal device. In the most common configuration, the BJT acts as an amplifier and can be used to amplify both voltage and current. BJTs can also be used in a switching configuration, where they are used to control the flow of current in a circuit. BJTs are versatile and can be used in a wide range of applications, from computers and consumer electronics to industrial and medical control systems.